1. Field of the Invention
The present invention relates to a liquid crystal display device and more specifically, to a method of manufacturing a liquid crystal display device such that a non-uniform alignment in an alignment layer is corrected to prevent the occurrence of non-uniform illuminance in the display.
2. Discussion of the Related Art
A liquid crystal display (LCD) apparatus is widely used as a display for notebook computers and portable TVs. An LCD generally includes an upper substrate, a lower substrate, a liquid crystal layer between the two substrates, and an upper alignment layer coated on the inner surface of the upper substrate, and a lower alignment layer coated on the inner surface of the lower substrate. The upper and lower alignment layers are provided with a desired alignment direction via a rubbing process, and a ground alignment state of the liquid crystal layer is determined by the alignment direction. However, after injection of the liquid crystal at room temperature, the effect of the flow of the liquid crystal is fixed in the injected liquid crystal layer, thus changing the desired alignment orientation.
The conventional solution to this problem is to use an aging process in which the liquid crystal cell is heated at a temperature higher than a nematic-isotropic transition temperature Tni. More specifically, the liquid crystal cell is heated while in an isotropic phase at a temperature greater than Tni. This process is called aging process, the aging temperature is 100. Thereafter, the liquid crystal cell is slowly cooled at room temperature while in the isotropic phase so that nucleation is initiated in the liquid crystal molecules that are adjacent to the surface of the substrate. This causes the liquid crystal molecules to be aligned in the desired alignment directions.
However, when the alignment layer is dried and baked after being coated on the substrate, non-uniform tilt angles occur in the alignment layer, which is caused by the non-uniform temperature distribution therein during drying and baking. Additionally, non-uniform tilt angles may also occur locally in the alignment layer if the rubbing process is performed incorrectly after baking of the alignment layer. Further, the alignment layer can be damaged by the injection of the liquid crystal after the rubbing process so that local non-uniformities in alignment may still occur in the alignment layer.
FIG. 1 is a cross-sectional view showing liquid crystal molecules 2 adjacent to a surface of the alignment layer 1, where region A indicates an abnormally aligned region having an improper alignment, and region B indicates a normally aligned region having the proper alignment. As shown in FIG. 1, the abnormally aligned region A and the normally aligned region B have liquid crystal molecules aligned with different tilt angles. Note that the tilt angles shown in FIG. 1 are exaggerated for easier explanation. The normally aligned region B generally has a tilt angle in the range of 4 to 5 degrees.
FIG. 2 is a graph which shows the transmittance T as a function of the driving voltage V in the abnormally aligned region A and the normally aligned region B. This graph shows the transmittance or illuminance difference X between abnormally aligned region A and the normally aligned region B at a grey level voltage Vg. This local non-uniform illuminance is caused by the non-uniform alignment in the alignment layer, thus, it cannot be solved by the conventional aging process.
Furthermore, when there are impurities in a pixel region of the alignment layer, abnormal alignment also occurs because of the impurities in the adjacent pixel regions as well as impurities in the pixel region itself. As described above, the abnormal alignment causes defects in images produced on the display.